Beam Alignment in mmWave V2X Communications: A Survey

The digital transformation within the automotive industry is accelerating towards an era dominated by autonomous vehicles, with vehicle-to-everything (V2X) communications being a fundamental enabler for this advancement. As vehicular networks evolve to meet the complex demands of autonomous driving, traditional communication systems encounter limitations in bandwidth and data transfer rates. Millimeter-wave (mmWave) communication emerges as a pivotal solution, offering the extensive bandwidth required for the high data throughput and low latency essential in modern vehicular communications. However, challenges loom, with beam alignment in mmWave V2X becoming a time-consuming process and the mmWave's blockage effect impeding consistent and reliable vehicular communication links. Therefore, the development of efficient, real-time, and robust beam alignment technology is crucial for mmWave V2X communication. In this paper, we present a comprehensive survey of beam alignment techniques in mmWave V2X communication. We explore various approaches including beam sweeping, angle of arrival (AoA)/angle of direction (AoD) estimation, black-box optimization, and side information. Subsequently, we introduce performance metrics for assessing beam alignment performance and compare the performance of four beam alignment methods under different metrics. Finally, we summarize the future research directions and challenges faced by beam alignment techniques in mmWave V2X communication, offering valuable insights for researchers in this field.